Disable button after first click and display confirmation box

Simple Code :- 

<asp:Button ID="btn" runat="server" Text="Start"
   UseSubmitBehavior="false"  
    OnClientClick="this.disabled=true;
      this.value = 'Please wait ...';"  onclick="btn_Click" />


With Confirm Message :

<asp:Button ID="btn" runat="server" Text="Start" 
   UseSubmitBehavior="false" 
      OnClientClick="if (!confirm('Certain??')) return; 
        this.disabled=true;this.value = 'Please wait ...';"      
 onclick="btn_Click" />